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The label DEFAULT where you expect an option-flag in the
ConfigSpecs list means that all the options not understood
by this mega-widget itself are passed-on to that subwidget(s). You can
be more precise with the Delegates facility.

In the example, -fg is defined to be an alias for
-foreground. Although: this specific alias is already defined
by Tk, so you don't have to specify this yourself.

Mega-widgets have more types of option-flags than derived widgets. The
manual-page Tk::configspec will help you on your way.